The present invention relates generally to microelectromechanical systems (MEMS) devices and operational methods, and more particularly, to the use of a zero rate output of a MEMS in-plane tuning fork gyroscope to electronically control the mechanical bias voltages to control the frequency difference between drive and sense resonant modes of the gyroscope.
Over the last decade, the resolution of silicon vibratory microgyroscopes has improved by almost ten times every two years. The improvements in noise floor can mainly be attributed to improved high aspect ratio microfabrication processes, better mechanical sensor design, and improved interfacing of the micromechanical sensor element with CMOS circuits. Current research is focused on development of microgyroscopes for automotive and consumer applications. However, existing microgyroscope performance must be improved by an order of magnitude if they are to be viable alternatives to fiber-optic gyroscopes. Low-cost, sub-degree per hour bias drift microgyroscopes will complement μ-gravity accelerometers to enable chip-scale navigation, and multi-axis motion analysis at micro-scale. In addition, such precision inertial measurement units (IMUs) are essential in micro-robotics, unmanned aerial/undersea vehicles and GPS-augmented navigation.
Micromachined Coriolis vibratory gyroscopes are ideal angular rate sensors for automotive applications, unmanned aerial vehicles, image stabilization in portable electronics and personal heading references, due to their low cost, light weight and small form factor. As MEMS gyroscopes attain inertial grade performance (i.e., sub-degree-per-hour rate resolutions and bias stabilities) the interface electronics that actuate, sense and control these micromechanical structures are key element in determining the over all performance of the micro-gyro system.
Micromachined gyroscopes constitute one of the fastest growing segments of the microsensor market. The application domain of these devices is quickly expanding from automotive to consumer and personal navigation systems. Examples include anti-skid and safety systems in cars, and image stabilization in digital cameras. Conventional MEMS gyroscopes do not meet the sub-degree-per-hour resolution and bias drift requirements needed in high precision applications such as inertial measurement units for GPS augmented navigation, robotics, unmanned surveillance vehicles, aircraft and personal heading references.
The majority of automotive and consumer electronics application require rate-grade performance, while high precision navigation-grade devices are suitable for inertial measurement units and high-end applications in aerospace and petroleum industry.
A multitude of applications exist in the automotive sector including navigation, anti-skid and safety systems, roll-over detection, next generation airbag and anti-lock brake systems. Consumer electronics applications include image stabilization in digital cameras, smart user interfaces in handhelds, gaming, and inertial pointing devices. IMUs are self-contained units that can perform accurate short-term navigation of a craft/object in the absence of global positioning system (GPS) assisted navigation. An IMU typically uses three accelerometers and three gyroscopes placed along their respective sensitive axes to gather information about an object's direction and heading. MEMS-based IMUs are increasingly being used in unmanned aerial/undersea vehicles for navigation and guidance. Since these remotely operated unmanned aerial/undersea vehicles experience diverse environments, in terms of shock, vibration and temperature, periodic calibration and reconfiguration of the IMU components becomes all the more important. Additionally, these are applications where power and area are premium. This calls for the development of smart angular rate sensors.
Vibratory micromachined gyroscopes rely on Coriolis-induced transfer of energy between two vibration modes to sense rotation. Micromachined gyroscopes are increasingly employed in consumer and automotive applications, primarily due to their small size and low power requirements. However, they are yet to achieve performance levels comparable to their optical and macro-mechanical counterparts in high-precision applications such as space and tactical/inertial navigation systems.
Conventional MEMS vibratory gyroscopes have yet to achieve inertial grade performance. The requirements for inertial grade devices are rate resolutions and bias stabilities better than 0.1°/h. To achieve this, a vibratory gyroscope must attain very high quality factors (>30,000), large sense capacitances (>1 pF), large mass (>100 μg), and large drive amplitude (>5 μm).
The Brownian motion of the structure represents the fundamental noise-limiting component of a vibratory gyroscope. This is generally discussed, for example, by Ayazi, F., in “A High Aspect-Ratio High-Performance Polysilicon Vibrating Ring Gyroscope,” Ph.D. Dissertation, University of Michigan, Ann Arbor (2001), and Ayazi, F. and Najafi, K., in “A HARPSS Polysilicon Vibrating Ring Gyroscope” IEEEIASME JMEMS, June 2001, pp. 169-179. By equating Brownian motion to the displacement caused by the Coriolis force, one can derive the mechanical noise equivalent rotation (MNEΩ) of the microgyroscope. This is expressed as
      MNE    ⁢                  ⁢    Ω    =                    1                  2          ⁢                                          ⁢                      q            Drive                              ·                                    4            ⁢                                                  ⁢                          k              B                        ⁢            T                                              ω              0                        ⁢            M                                ⁢          BW      
This equation indicates that the mechanical noise floor varies inversely with the drive amplitude (qDrive), the square root of the resonant drive frequency (ω0), and square root of the effective mass in the sense direction (M). Matching the resonant frequencies of the sense and the drive mode improves this resolution by a factor of √{square root over (QSense)}.
There is a need for improved tuning fork gyroscopes and angular rate sensors employing same that provide for electronic bandwidth control.